1. Field of the Invention
The present invention generally relates to a wireless communication system. More particularly, the present invention relates to a method and apparatus for transmitting and receiving an Acknowledgment/Negative Acknowledgment (ACK/NACK) for Hybrid Automatic Repeat reQuest (HARQ).
2. Description of the Related Art
There are two main error control schemes in a data transmission system, Forward Error Correction (FEC) and Automatic Repeat reQuest (ARQ). An FEC system attempts to correct errors in received data. If the error correction is successful, the correct data is decoded. If the error correction has failed, the wrong data is provided to a user or the data is lost. In an ARQ system, a transmitter transmits data using an FEC code with a good error correction capability and if an error is detected from the received data, a receiver requests retransmission to the transmitter.
FEC is relatively less efficient in a good channel environment and if error correction has failed, it decreases system reliability. On the other hand, ARQ has the advantages of high system reliability and efficient transmission with low redundancy, but it causes frequent retransmission requests in a poor channel environment, thus significantly decreasing system efficiency. To overcome these shortcomings, HARQ was proposed by combining FEC and ARQ in an appropriate manner.
HARQ is a scheme that attempts to correct errors in a received coded data (a HARQ packet). It determines from a simple error detection code such as a Cyclic Redundancy Check (CRC) whether to request retransmission of the HARQ packet. After determining the presence or absence of errors in the received HARQ packet, the receiver feeds back an ACK or NACK to the transmitter. The transmitter retransmits the HARQ packet or transmits a new HARQ packet according to the ACK or NACK.
The receiver uses appropriate radio resources for the ACK/NACK transmission. The ACK/NACK is transmitted on a few subcarriers in an Orthogonal Frequency Division Multiplexing (OFDM) wireless communication system and on a predetermined code channel in a Wideband Code Division Multiple Access (WCDMA) system. In general, HARQ packets are transmitted simultaneously to a plurality of users for one Transmission Time Interval (TTI). Therefore, ACKs/NACKs are also transmitted simultaneously for the HARQ packets.
When a Node B allocates downlink data channels to User Equipments (UEs), it also allocates control channel resources in which the UEs will transmit ACK/NACKs for the downlink data channels. For uplink data transmission, the Node B receives uplink packet data from the UEs on uplink data channels and then transmits ACK/NACKs for the packet data in resources agreed between the Node B and the UEs.
In general, limited resources are available to a system and the system resources should be appropriately divided for channels including data channels and ACK/NACK CHannels (ACKCHs). Hence, it is significant to allocate as much resources as needed for a given TTI to the ACKCHs. To describe the resource allocation, an Enhanced Universal Terrestrial Radio Access (EUTRA)-OFDM downlink frame structure is shown in FIG. 1, by way of example. UTRA is the future-generation mobile communication standards of the 3rd Generation Partnership Project (3GPP).
Referring to FIG. 1, a system bandwidth 101 is 10 MHz and a total of 50 Resource Blocks (RBs) 102 are defined in the system bandwidth 101. Each RB 102 includes 12 subcarriers and each 1-ms TTI 105 has 14 OFDM symbol intervals 104. One downlink data channel can be formed with one or more RBs.
In the downlink frame structure of FIG. 1, up to 50 downlink data channels can be scheduled at the same time for one TTI 105. Accordingly, up to 50 uplink ACKCHs are required. In real implementation, 10 or 20 data channels are scheduled for one TTI on average and as many uplink ACKCHs are needed for the data channels. Since the number of available ACKCHs is very different from the average number of actually used ACKCHs, an efficient resource allocation is important.
If the Node B explicitly notifies UEs of ACKCHs established for data channels in every TTI, only as many ACKCHs as needed can be allocated for the TTI. Thus, for downlink data transmission, the UEs transmit ACK/NACKs in the notified ACK/NACK resources. For uplink data transmission, the UEs detect ACK/NACKs transmitted by the Node B from ACK/NACK resources signaled by the Node B. Hence, it is significant to reduce the amount of resources for the signaling, i.e. signaling overhead. The explicit signaling of information about ACK/NACK resources from the Node B to the UEs for each TTI results in a large signaling overhead.
Accordingly, there exists a need for optimizing the amount of resources allocated to ACKCHs and the overhead of signaling the ACK/NACK resources in order to increase system capacity through efficient use of radio resources.